A high frequency filter, e.g. a ceramic or SAW filter has previously been used in connection with a transformer or a mixer to filter out interference signals or undesired mixing results.
FIG. 1 shows a transformer construction according to the conventional approach. The transformer is realized on a multilayer printed circuit board so that the primary 1 and the secondary 2 and 3 are located in different conductor layers. The coupling between the primary and the secondary is effected through a magnetic field. The transformer circuit frequency band is narrowed with the aid of capacitors 4 and 5, whereby the construction forms a resonant circuit.
FIG. 2 shows the frequency response of a transformer according to the conventional approach. The graph 7 represents the power distribution from port 1 into ports 2 and 3. The graph 6 illustrates the return loss of the input port. In FIG. 2 the frequency (MHz) is on the horizontal axis and the attenuation (dB) on the vertical axis. The power is equally divided between the ports 2 and 3; therefore the graph 7 comprises two superimposed curves. Due to the construction of the transformer, however, the signals have a phase difference of 180 degrees. FIG. 2 shows that it is a rather wideband transformer arrangement.
A separate high frequency filter entails costs and it consumes space on the printed board, which poses a problem when aiming at smaller total areas.
The object of the present invention is to provide a transformer with which a high frequency filter can be combined.